It has heretofore been suggested to make microvoid-containing polymer particles to serve as hiding or opacifying agents in coating and molding compositions. Among the various procedures heretofore used, organic solvents and blowing agents occupy a prominent place therein.
Kreider U.S. Pat. No. 3,819,542, though containing no disclosure of the production of microvoid-containing polymer particles to serve as opacifying agents in coating compositions, is of background interest to show the use of organic solvents in a latex coating composition to produce a cellular film on drying. More specifically Kreider uses a mixture of a primary organic solvent, such as xylene, immiscible with the aqueous phase of the latex coating composition and a secondary organic solvent, such as propylene glycol, at least partially water-miscible and having lesser volatility and lesser solvating capacity for the polymer in the latex than the primary solvent. Upon evaporation of the primary solvent, a cellular film is obtained, the secondary solvent increasing the opacification.
Kershaw et al, U.S. Pat. No. 3,891,577, prepares a vesiculated polymer by converting to a solid polymer a liquid medium containing dispersed therein particles of another polymer swollen by a liquid swellant, the liquid swellant then being at least partially removed from the disperse polymer particles. The liquid medium may be converted to a solid by removal of solvent, e.g., from a solution of a solid polymer, or preferably by polymerization of a monomer or comonomers or an oligomer or a mixture of these. Optionally a dissolved polymer may be present in the liquid to be polymerized. Solidification of the liquid in which the swollen particles are dispersed and removal of the swellant is then carried out to provide the vesiculated polymer, which may be in massive form, as a film, or in the form of a coating applied to a substrate.
In another Kershaw embodiment, the dispersion of swollen polymer in the liquid medium may itself be dispersed in a further liquid in which it is insoluble. The further liquid is referred to as the suspending liquid. Solidification of the medium is then carried out and after separation of the granules so formed from the suspending liquid, liquid swellant may be removed from the swollen polymer to provide vesiculated polymer in granular form. Alternatively, when, for example, the vesiculated granules are to be used in a coating composition with which the suspending liquid is compatible, the granules formed by solidification of the medium may be incorporated into the composition as a slurry in at least part of the suspending liquid. On applying the composition to a substrate, formation of a coating film and removal of swellant from the swollen disperse polymer to form the vesicles within the granules then take place concurrently.
Whereas this U.S. Pat. No. 3,891,577 discloses a wide range of swellable disperse polymers including those containing an acid group that would be swollen by water having a pH greater than 7, e.g. water containing ammonia, and whereas it mentions that aqueous emulsion polymerization can be used to make the swellable polymer particles in a latex that can be added to a water-miscible medium to be solidified or that can be dried to form particles that can be redispersed in such a medium, nevertheless, the patent primarily depends on the process of dispersion (non-aqueous) polymerization to prepare the swellable polymer particles as pointed out in column 5, lines 28 to 53, and elsewhere in the general description, as well as in most, if not all the working examples of the patent. Such procedures are generally complicated and involve the use of organic solvents with the attendant health and fire hazards as well as the need to provide expensive solvent exhaust and/or recovery systems.
Kurth et al. U.S. Pat. No. 3,875,099 discloses preparation of sequential acrylic polymers containing 0.5-2.5% of an alpha, beta-unsaturated carboxylic acid. The bulk of the acid is introduced in the early portion of the polymerization. Of the 11 examples, only Ex. 1 superficially resembles the film-forming thickener aspect of the present invention. There is about 0.5% acid monomer over all, the first stage monomers containing about 13% methacrylic acid. The core/sheath weight ratio is in the neighborhood of 1/25. The core and sheath monomers are basically the same in this example (about 50:50 butyl acrylate methyl methacrylate), but the acid in the core confers a T.sub.i of about 30.degree.-35.degree. C., the sheath having a calculated T.sub.i of about 5.degree.-10.degree. C. Example 1 of Kurth et al. involves the use of a large amount of surfactant and a small amount of peroxy initiator, both of which are believed to normally result in extremely small latex particles. In our experience, this recipe would be expected to give core particles of about 0.04-0.05 micro average diameter, with the overall core-shell particle diameter being about 0.1-0.15 micron. The soft core, the small particle size, and the high temperature polymerization each may contribute to incomplete encapsulation. As shown by a comparative example herein below, the first stage of the Kurth et al. Ex. 1 is not adequately encapsulated.
Canadian Pat. No. 888,129 discloses the preparation of a two-stage latex dispersion of particles having a core containing a blowing agent and an encapsulating layer. The particles are subsequently heated to develop gas and foam the particles.
Krieg et al, U.S. Pat. No. 3,914,338, discloses the production of opalescent polymer particles for use in polymethyl methacrylate molding compositions consisting of a core of a crosslinked styrene emulsion polymer of a particle diameter of at least 0.8 micron and having grafted thereon, as by a subsequent stage of emulsion polymerization, a sheath of a methyl methacrylate polymer. Opalescence apparently depends on the different refractive indices of the core and sheath.
Fantl et al, U.S. Pat. No. 3,401,134 discloses that cellulose ethers and water-soluble gums have been used to thicken aqueous coating compositions based on various polymer latices to thicken the composition for application, as by brushing, rolling, or the like. Fantl et al disclose that improved aqueous polymer dispersions are obtained by delaying the incorporation of acid monomer into the copolymer until at least 70% of the other monomers have been polymerized, asserting that the acid mer units are thereby oriented to the surface of the polymer particles and the acid copolymer thus obtained has the property of increasing in viscosity when the addition of a base, such as ammonia, raises the pH to 8 to 12.